The present invention relates to an objective lens which is used or recording data to and/or reproducing data from a plurality of types of optical discs having different data recording densities and cover layer thicknesses.
There exist a plurality of types of standards regarding optical discs having different data recording densities and different thicknesses of cover layers (protective layers). For instance, DVD (Digital Versatile Disc) has higher data recording density than that of CD (Compact Disc) and has a thinner cover layer than that of CD. Therefore, when the objective lens is used for the plurality of types of optical discs, a proper beam spot corresponding to the data recording density of an optical disc being used has to be formed on the optical disc by changing NA (Numerical Aperture) of a light beam while properly correcting spherical aberration which changes depending on the thickness of the cover layer.
For example, for recording/reproducing operation of the optical disc having a relatively thinner cover layer and higher data recording density, the NA has to be increased in comparison with the NA of an optical system designed specifically for the optical disc having a relatively thick cover layer and lower data recording density. Since the beam spot diameter gets smaller as a wavelength of the beam gets shorter, a laser light source of an oscillation wavelength of 635-665 nm is used for an optical system for DVD. The wavelength for DVD is shorter than that (780-830 nm) of the optical system for CD. Therefore, optical devices of recent years used for recording data to and/or reproducing data from the plurality of types of optical discs are generally equipped with a light source unit capable of emitting laser beams of different wavelengths.
Objective lenses used for properly converging the laser beam on data recording surfaces of the plurality of types of optical discs having different cover layer thicknesses are disclosed in the following publications.
Japanese Patent Provisional Publication No. 2001-243651 (hereinafter referred to as “document #1”) discloses an objective lens configured to have a diffracting structure having a plurality of annular zones with small steps on one surface of the objective lens. Two types of collimated beams having different wavelengths are used to be incident on the objective lens.
Also, each of Japanese Patent Provisional Publication No. HEI09-179021 (hereinafter referred to as “document #2”) and Japanese Patent Provisional Publication No. 2001-76367 (hereinafter referred to as “document #3”) discloses a configuration of an optical system in which two types of beams of different wavelengths are switched and used for two types of optical discs, similarly to the configuration disclosed in the document #1.
In the optical system disclosed in the document #2, a collimated beam is incident on the objective lens when the optical disc of higher data recording density (e.g., DVD) is used, while a diverging beam is incident on the objective lens when the optical disc of lower data recording density (e.g., CD) is used. In the optical system disclosed in the document #3, a diverging angle of a diverging beam incident on the objective lens, which has a surface on which a diffracting structure having a plurality of annular zones with small steps, is set smaller in the recording/reproducing operation of CD than in the recording/reproducing operation of DVD.
Incidentally, the objective lens, used for recording data to and/or reproducing data from the plurality of types of optical discs, is required not only to correct the aforementioned spherical aberration but also to satisfactorily correct a coma which occurs when the beam is obliquely incident upon the objective lens with respect to an optical axis of the objective lens, in converging each laser beam (corresponding to each optical disc) on the data recording surface of each optical disc. However, in cases where the same magnification of the objective lens is employed for both of the DVD and CD (as in the document #1) or a magnification of the objective lens in use of the CD is smaller than a magnification of the objective lens in use of the DVD (as in the document #2), it is difficult to satisfactorily suppress the coma (occurring when the beam is obliquely incident upon the objective lens with respect to the optical axis) for each of the optical discs.
For this reason, in the conventional optical device, the objective lens is configured only to achieve a balance between comas occurring in use of optical discs of different types, depending on the purpose of the optical device. Specifically, the tolerance for aberrations becomes lower in the recording/reproducing operation for the DVD having higher data recording density. Thus, in many cases, the objective lens is designed to reduce the coma in use of the DVD as much as possible even if the coma in use of the CD increases to some extent. In the optical device disclosed in the document #3, a distance between a light source and a disc surface of the CD and a distance between the light source and a disc surface of the DVD are set equal to each other, and a magnification in use of the CD is slightly larger than that in use of the DVD. With this structure, the amount of coma can be corrected to some extent for each of the optical discs. However, the coma can not be suppressed effectively since a difference between the magnifications in use of the CD and DVD is small.
Thus, an objective lens that is capable of improving performance for recording data to and/or reproducing data from the plurality of types of optical discs is desired.